"HTML5" in this article sometimes refers not only to the HTML5 specification, which does not itself define ways to do animation and interactivity within web pages,[2] but to HTML5 and related standards like JavaScript or CSS 3. Animation via JavaScript is also possible with HTML 4.

Web browsers cannot render Flash media themselves, instead it is rendered primarily via the proprietary but freely available Adobe Flash Player. Until 2008, there was no official specification which was allowed to be used to create an alternative player. Alternative players have been developed before 2008, but they support Flash to a lesser degree than the official one.[56]

All web browsers support HTML and other Web standards to various degree. Adobe released a tool that converts Flash to HTML5,[59] and in June 2011, Google released an experimental tool that does the same.[60][61]

As of December 2013[update], versions of browsers such as Chrome, Firefox, Internet Explorer, Opera, and Safari implement HTML5 to a considerable degree. However, some portions of the HTML5 specification were still being implemented by browser makers.

As of January 2015, YouTube defaults to HTML5 players to better support more devices.[62]

Until 2008, the use of Flash was covered by restrictive licenses. The conditions prohibited use of the specification to develop any software (including players) which could render or read (and thus convert) SWF files, and required the output SWF files to be compatible with Adobe's players.[63]

In 2008, restrictions on use of the SWF and FLV/F4V specifications were dropped, and some specifications were released.[64] However, the "SWF File Format Specification Version 10" allegedly did not contain all the needed information, did not contain much information that hasn’t been previously known by the community,[65] and itself could not be copied, printed out in more than one copy, distributed, resold or translated, without written approval of Adobe Systems Incorporated.[66]

Various people have praised Flash over the years for rendering consistently across platforms. Constructing sites in Flash is a way to prevent code forking, whereby different versions of a site are created for different browsers.[67][68]

Speaking at 'Adobe Max' in 2011, Itai Asseo likewise said that, unlike HTML 5, Flash offers a way to develop applications that work across platforms. HTML 5, he said, is currently implemented differently (if at all) by different browsers. Although the Flash browser plugin is not supported on the Apple iPhone OS, Flash applications can be exported to Adobe AIR, which runs on that operating system as a native application. In the same talk, Mr. Asseo lamented the return to another browser war (as seen in the late 1990s). If Flash falls out of favor, he said, web developers will either have to develop many different versions of their web sites and native applications to take into account different HTML 5 implementations, deny access to browsers that do not support their version of HTML, or dramatically reduce the functionality of their sites in order to deliver content to the least-advanced browser.[69]

Constructing Flash websites using Adobe tools is relatively easier than with integrated development environments for CSS, HTML, and JavaScript;[70] however, many of Adobe's tools are expensive and proprietary software.[70]

Tools for HTML5 are just starting to come to market; in the meantime, Adobe has released a first version of a Flash-to-HTML5 conversion tool for existing content[71] and are working on creating new tooling for HTML5 as well, like Adobe Edge.[72]

Because HTML5 is an open format, tools like those for Flash can be built for it, too. Applications like Hype[73] and Adobe Edge are already on the market.

Some users, more so those on OS X and Linux, have complained about the relatively high CPU usage of Flash for video playback.[74][unreliable source?] This was partially because the Flash plugin did not use the GPU to render video. Adobe has responded to some of those criticisms in the 10.1 and 10.2 releases of the Flash plugin by offloading H.264 video decoding to dedicated hardware and by introducing a new video API called Stage Video.[75][unreliable source?] In addition, the use of the newer ActionScript 3.0 inside Flash movies instead of the older ActionScript 2.0 improves code execution speed by a factor of around 10. But older websites that use ActionScript 2.0 will not benefit from this.[76] The software routines written by developers can also affect the performance of applications built in Flash, reasons that would affect HTML5 animations as well.[77]

An HTML vs Flash comparison in 2010 listed non-Flash technologies as being slower than Flash when used for non-video animations,[78] but in January 2012 some of them were faster.[79]

Flash has the ability to specify measurements in sub-pixel increments. This can result in a crisper and generally more pleasant appearance of Flash web sites. When confronted with CSS and HTML measurements on a sub-pixel scale, web browsers will round either up or down, depending on the browser, which leads to inconsistency and unreliability in the display of those pages.[70]

Flash offers webcam support, while HTML and related technologies did not until recently.[clarification needed]

There are, however, people working on adding "device support" (device API) to the HTML5 specification, which would allow for videoconferencing, access to webcams, microphones, USB-thumbdrives and other USB- or serial devices.[80][81]

HTML5 does not include any digital rights management functionality. Implementations can support DRM outside the scope of HTML, for example in codecs.[82] The proposal to add DRM features to HTML5 itself has been criticised by those who consider openness and vendor-neutrality (both server- and client-side) one of the most important properties of HTML, because DRM is incompatible with free software,[83][84] and in the proposed form potentially not more vendor-neutral than proprietary plug-ins like Flash.[85][86]

Both Flash and HTML text can be read by screen readers. However, special care must be taken to ensure Flash movies are read correctly. For example, if a Flash movie is set to repeat indefinitely, this can cause a screen reader to repeat the content endlessly. Selecting the "Make object accessible" check box in Adobe Flash Professional will create a text-only version of the object for screen readers. It will also hide any motion from the screen reader.[87] Since Flash content is usually placed on a single webpage, it appears as a single entry in search engine result pages, unless techniques like deep linking are used with libraries like SWFAddress to provide multiple links within Flash websites and web applications. User interface widgets in Flash objects don't always behave like their host native counterparts. Keyboard, mouse and other accessibility shortcuts may not work unless the webpage developer explicitly adds support for it.

Both Flash content and HTML content can be indexed by Google, Yahoo!, and Bing, although bi-directional text (e.g. Arabic, Hebrew) is not supported by Google.[88][89] Yahoo! added support for indexing Flash sites in 2008, although Google had been able to index them for several years before that. Bing added support for Flash sites in 2010.